Piero G. Pavan

Viscoelastic Response of the Periodontal Ligament : An Experimental-Numerical Analysis

A viscoelastic constitutive model for the periodontal ligament (PDL) capable of accounting for large strains, anisotropy, and inelastic time-dependent effects was developed. Anisotropy characteristics are determined by the composite nature of the tissue and, in particular, by the distribution of collagen fibres. Time-dependent viscous phenomena are due to microstructural modifications during loading, such as fluid fluxes moving through the solid matrix and the internal rearrangement of fibers and constitutive adaptation. The viscoelastic model presented here was implemented in a general purpose finite element code. In vitro experimental tests were carried out on the PDL specimens of adult pigs to obtain stress-relaxation and cyclic stress-strain curves. The comparison of experimental and numerical results revealed good correspondence and confirmed the capability of the formulation assumed to properly interpret the viscoelastic behavior of the PDL.

A visco-hyperelastic-damage constitutive model for the analysis of the biomechanical response of the periodontal ligament.

The periodontal ligament (PDL), as other soft biological tissues, shows a strongly non-linear and time-dependent mechanical response and can undergo large strains under physiological loads. Therefore, the characterization of the mechanical behavior of soft tissues entails the definition of constitutive models capable of accounting for geometric and material non-linearity. The microstructural arrangement determines specific anisotropic properties. A hyperelastic anisotropic formulation is adopted as the basis for the development of constitutive models for the PDL and properly arranged for investigating the viscous and damage phenomena as well to interpret significant aspects pertaining to ordinary and degenerative conditions. Visco-hyperelastic models are used to analyze the time-dependent mechanical response, while elasto-damage models account for the stiffness and strength decrease that can develop under significant loading or degenerative conditions. Experimental testing points out that damage response is affected by the strain rate associated with loading, showing a decrease in the damage limits as the strain rate increases. These phenomena can be investigated by means of a model capable of accounting for damage phenomena in relation to viscous effects. The visco-hyperelastic-damage model developed is defined on the basis of a Helmholtz free energy function depending on the strain-damage history. In particular, a specific damage criterion is formulated in order to evaluate the influence of the strain rate on damage. The model can be implemented in a general purpose finite element code. The accuracy of the formulation is evaluated by using results of experimental tests performed on animal model, accounting for different strain rates and for strain states capable of inducing damage phenomena. The comparison shows a good agreement between numerical results and experimental data.

Constitutive modelling of inelastic behaviour of cortical bone.

A visco-elasto-plastic constitutive model is formulated for investigating the mechanics of cortical bone tissue, accounting for an anisotropic configuration and post-elastic and time-dependent phenomena. The constitutive model is developed with reference to experimental data obtained from literature on the behaviour of cortical bone taken from multiple samples. Regarding the constitutive model, a specific procedure based on a coupled deterministic and stochastic method is applied in order to determine the values of the constitutive parameters with regard to human samples. The procedure entails processing of data deduced from mechanical tests to achieve relationships between permanent and total strain, elastic modulus and strain rate, and creep elastic modulus and time. Numerical results obtained by using a finite element model are compared with tensile experimental data on cortical bone including the post-elastic range and creep phenomena. The model shows an excellent capability to describe the tensile behaviour of the cortical bone for the specific mechanical condition analysed.

A Transversally Isotropic Elasto-damage Constitutive Model for the Periodontal Ligament

A numerical formulation of an elasto-damage constitutive model was developed and implemented in a finite element software to investigate the biomechanical response of the periodontal ligament (PDL). The mathematical framework accounts for the description of large strains, anisotropy and inelastic phenomena. The anisotropic mechanical response is caused by the spatial orientation of the sub-structures of the tissue, such as collagen fibres. Inelastic behaviour, induced by high level strains, is modelled by means of damage models. In vitro experimental testing on PDL samples from pigs was performed to obtain tensile stress-strain curves. A finite element analysis is presented in order to define a general numerical approach. A comparison of numerical and experimental data is provided in order to show the reliability and effectiveness of the formulation assumed.

Decellularized human skeletal muscle as biologic scaffold for reconstructive surgery

Engineered skeletal muscle tissues have been proposed as potential solutions for volumetric muscle losses, and biologic scaffolds have been obtained by decellularization of animal skeletal muscles. The aim of the present work was to analyse the characteristics of a biologic scaffold obtained by decellularization of human skeletal muscles (also through comparison with rats and rabbits) and to evaluate its integration capability in a rabbit model with an abdominal wall defect. Rat, rabbit and human muscle samples were alternatively decellularized with two protocols: n.1, involving sodium deoxycholate and DNase I; n.2, trypsin-EDTA and Triton X-NH4OH. Protocol 2 proved more effective, removing all cellular material and maintaining the three-dimensional networks of collagen and elastic fibers. Ultrastructural analyses with transmission and scanning electron microscopy confirmed the preservation of collagen, elastic fibres, glycosaminoglycans and proteoglycans. Implantation of human scaffolds in rabbits gave good results in terms of integration, although recellularization by muscle cells was not completely achieved. In conclusion, human skeletal muscles may be effectively decellularized to obtain scaffolds preserving the architecture of the extracellular matrix and showing mechanical properties suitable for implantation/integration. Further analyses will be necessary to verify the suitability of these scaffolds for in vitro recolonization by autologous cells before in vivo implantation.

Investigation of bone inelastic response in interaction phenomena with dental implants

OBJECTIVES The aim of the paper is to analyze the effects of misfits in multi-implant oral prostheses caused by defects in manufacturing of bar connecting implants. The consequent stress-strain state on peri-implant bone tissue must be carefully considered because of the significant effects induced. MATERIALS AND METHODS The case of a two-implant prosthesis connected by a titanium cast bar in the pre-molar mandible region is investigated. The complex geometry requires the use of refined finite element model. In consideration of the action induced on the bone tissue, a specific constitutive model that includes inelastic behavior is implemented. The linear misfits considered, estimated with reference to experimental works already present in the literature, induce relevant strains in the peri-implant bone tissue up to a post-elastic phase. Relaxation phenomena are also evaluated. The interaction between the bone and implant is modeled by using contact elements to represent possible detachments at the bone-implant interface. RESULTS The response of the bone material is reported with regard to the stress/strain field induced, evaluating the inelastic behavior in terms of plastic and relaxation responses. The effects on the peri-implant bone tissue at the interface are evaluated. SIGNIFICANCE The analysis confirms that the interaction phenomena between the multi-implant oral prostheses and bone induced by the misfit defects of normal intensity induce significant strain in the bone tissue and inelastic phenomena must be taken into account. Local permanent strains of bone tissue and relaxation phenomena represent short-term tissue response for a correct interpretation of the real biomechanical behavior of multiple implant frames.

Modelling of mandible bone properties in the numerical analysis of oral implant biomechanics

The biomechanical efficiency of oral implants is deeply influenced by mechanical properties of cortical and trabecular bone in the jaw and, in particular, in the peri-implant region. When the mechanical response of the implant-bone system is analysed by means of numerical models, the effective mechanical properties of bone and the possible change as a function of spatial position must be carefully considered. The procedure presented provides for the attribution of the mechanical properties of bone, considered as anisotropic elastic material, as a function of the spatial position making use of Fourier series and polynomial functions. The procedure is implemented in a general purpose finite element software, adopted to develop biomechanical analyses of prosthetic systems. This procedure allows for an accurate representation of bone tissue properties. Results pertaining to the analysis of commercial oral implants show the potential of the method adopted.

Improvement of diaphragmatic performance through orthotopic application of decellularized extracellular matrix patch.

Muscle tissue engineering can provide support to large congenital skeletal muscle defects using scaffolds able to allow cell migration, proliferation and differentiation. Acellular extracellular matrix (ECM) scaffold can generate a positive inflammatory response through the activation of anti-inflammatory T-cell populations and M2 polarized macrophages that together lead to a local pro-regenerative environment. This immunoregulatory effect is maintained when acellular matrices are transplanted in a xenogeneic setting, but it remains unclear whether it can be therapeutic in a model of muscle diseases. We demonstrated here for the first time that orthotopic transplantation of a decellularized diaphragmatic muscle from wild animals promoted tissue functional recovery in an established atrophic mouse model. In particular, ECM supported a local immunoresponse activating a pro-regenerative environment and stimulating host muscle progenitor cell activation and migration. These results indicate that acellular scaffolds may represent a suitable regenerative medicine option for improving performance of diseased muscles.

A constitutive model for the mechanical characterization of the plantar fascia

A constitutive model is proposed to describe the mechanical behavior of the plantar fascia. The mechanical characterization of the plantar fascia regards the role in the foot biomechanics and it is involved in many alterations of its functional behavior, both of mechanical and nonmechanical origin. The structural conformation of the plantar fascia in its middle part is characterized by the presence of collagen fibers reinforcing the tissue along a preferential orientation, which is that supporting the major loading. According to this anatomical evidence, the tissue is described by developing an isotropic fiber-reinforced constitutive model and since the elastic response of the fascia is here considered, the constitutive model is based on the theory of hyperelasticity. The model is consistent with a kinematical description of large strains mechanical behavior, which is typical of soft tissues. A fitting procedure of the constitutive model is implemented making use of experimental curves taken from the literature and referring to specimens of human plantar fascia. A satisfactory fitting of the tensile behavior of the plantar fascia has been performed, showing that the model correctly interprets the mechanical behavior of the tissue in the light of comparison to experimental data at disposal. A critical analysis of the model with respect to the problem of the identification of the constitutive parameters is proposed as the basis for planning a future experimental investigation of mechanical behavior of the plantar fascia.

Investigation of viscoelastoplastic response of bone tissue in oral implants press fit process

According to the standard surgical protocols, the press fit is obtained inserting an implant in a drilled hole that is provided with a lower diameter. In this way, it is induced a relevant strain state in the peri-implant bone that favors the primary stability of the implant. Experimental evaluation of this phenomenon is very difficult and does not offer a complete set of information. A numerical analysis is adopted to describe the mechanical phenomena occurring in the peri-implant tissue. At this purpose, suitable constitutive models are adopted for the bone tissue for the evaluation of plastic and viscous effects caused by the real strain field induced. Specific numerical procedures are developed to model the press fit action of an implant against the surrounding bone tissue and the subsequent viscoelastoplastic effects determined. The results of the numerical analysis make it possible to estimate the deformation caused by the insertion of the implant and the evolutionary trend after insertion by considering the inelastic time-dependent behavior of bone tissue in peri-implant region. According to the viscous characteristic of the bone tissue, the numerical analyses show a stress relaxation in the order of 30% around the implant.